1. Field of the Invention
The present invention relates to an illuminating method and an illuminating device for producing an illuminating light beam for a display device, a measuring device, a microscope, an exposure device, or the like.
2. Description of the Related Art
Conventionally, as a light source for illuminating light used in an illuminating device of, for example, a projection type liquid crystal display or a measuring device, an incoherent light source such as a lamp or light emitting diode (LED) has been used.
On the contrary, trials to use laser light from a laser light source, such as a solid-state laser, a gas laser, or a semiconductor laser, for illuminating light have been carried out. The laser light is superior in directionality and has high intensity, and is a light beam of high coherence. However, speckle resulting from high coherence becomes the most difficult technical problem.
For example, a semiconductor laser is a light source which has a very high photoelectric conversion efficiency and emits laser light with excellent directionality. However, it has been hardly used as an illuminating light source because of the problem of speckle due to high coherence.
In the 1970s, although studies of a display (hereinafter referred to as a laser display) using laser light were carried out at various places, in addition to problems of insufficient output of the light source and the modulation method, the problem of speckle was one of the problems which became obstacles against realization in practical use.
In recent years, the development of elementary techniques as key components of a laser display, such as a high output laser using wavelength conversion of a solid-state laser, semiconductor lasers capable of oscillating three primary colors of red (R), green (G), and blue (B), and a spatial modulator (light valve) using a liquid crystal or a micromachine, have been made at a high pace.
It is known that the contrast of speckle highly depends on the spectrum width of a light beam emitted from a light source. When the center wavelength of the light beam is .lambda..sub.0, the full width at half maximum of the wavelength spectrum is .DELTA..lambda., the spectrum width of the wave number is W, and the spectrum of the wave number is S(k), that is, ##EQU2##
an object is illuminated with this light, and in an optical system for forming an image of the object, when the light path length difference from one point on the object to its corresponding image has fluctuation of a standard deviation of .sigma..sub.Z, it is known that the standard deviation .sigma..sub.I of speckle intensity to the mean value &lt;I&gt; of illumination intensity is obtained by ##EQU3##
(see G. Parry, "Laser Speckle and Related Phenomena", pp. 93, Springer-Verlag, 1984).
When N speckle patterns which are incoherent from each other (that is, they do not interfere with each other) and have no correlation, are superimposed, the sum becomes an intensity sum of the respective speckle patterns. At this time, the contrast of speckle is lowered to 1/N.
For example, when N optical fibers are bundled, and the length of each of the optical fibers is changed by a length not less than a coherence length, it becomes possible to neglect the interference between the respective optical fibers. The speckle at this time is superposition of intensities of speckle patterns I.sub.1, I.sub.2 . . . I.sub.N formed by the respective optical fibers. Thus, the contrast of speckle is lowered by averaging (unifying).
This will be described with reference to a document (E. G. Rawson, J. W. Goodman, R. E. Norton, J.O.S.A. 70, 968-976,1980, Appendix).
First, when the spatial mean intensity of speckle patterns from each optical fiber is arbitrary, and the contrast of each speckle is L, the following expression is established. ##EQU4##
At this time, the contrast C of speckle produced as a result of superposition, which is expressed by the following expression, and the multiplication Neff (N.sub.eff) of effective speckle are obtained. The contrast C of speckle is expressed by ##EQU5##
and the ensemble mean intensity of a total sum is obtained as a sum of each ensemble mean intensity like the following expression. ##EQU6##
At this time, variance .sigma..sub.I.sup.2 expressed by ##EQU7##
When the correlation coefficient of a k-th speckle and an L-th speckle pattern is .rho..sub.kL, the following expression is established. ##EQU8##
Here, from the relation of the assumption of L-th contrast and ##EQU9##
the following expression is obtained. ##EQU10##
Thus, the contrast of the sum of patterns of all speckles becomes ##EQU11##
That is, the multiplicity Neff (N.sub.eff) of effective speckle becomes ##EQU12##
Here, if N speckle patterns are incoherent with each other, ##EQU13##
and at this time, ##EQU14##
is established. That is, if N speckle patterns which are incoherent and have equal intensity are superimposed on each other, the contrast becomes 1/N.
Fujii et al studied the contrast of speckle produced on an image of a random phase object by spatially partial coherent illumination (see H. Fujii, T. Asakura, opt. Comm. 12, 32-38, 1974).
According to that study, the speckle contrast C is expressed by ##EQU15##
Here, .GAMMA. is the correlation function of the electric field amplitude between two points of the object, K is the amplitude transfer function, and R is the correlation function of the phase of the object.
Concerning a typical numerical value example, when these functions are subjected to a rough calculation in a triangle region, the result as described below is obtained.
That is, the resolution km of the amplitude impulse response of an optical system is made km 1.8 .mu.m, and the spatial coherence length L.sub.C is made L.sub.C =15 .mu.m. The roughness of a rough surface of an object is expressed by a correlation length .alpha.. When each parameter is changed such that the correlation length .alpha. is 0.1 .mu.m, 0.5 .mu.m, 1.0 um, 2.0 um, 5.0 .mu.m, and 10.0 .mu.m, and the contrast of speckle is plotted on the vertical axis, a graph as shown in FIG. 1 is obtained. Here, the horizontal axis &lt;.phi..sup.2 &gt; of the graph of FIG. 1 is a root square mean of depth.
That is, in the case where the object has a sufficiently rough surface, &lt;.phi..sup.2 &gt; is increased, and .alpha. is decreased. It is conceivable that a normal screen has a sufficiently rough surface. From the graph shown in FIG. 1, the contrast of speckle in this calculation example becomes about 0.85.
In recent years, speckle (speckle pattern) has become a serious problem in the field of semiconductor exposure devices, and measures to this have been taken. The background of this is that an excimer laser as a short wavelength light source has been introduced to improve resolution.
In a semiconductor exposure device, as controls of coherence, that is, as measures to the speckle, for example, as shown in FIG. 2, there is proposed a coherence reducing method in which a fly eye lens 71 constituted by elements with different lengths is used, a lens 72 is disposed between an emitting side end of the fly eye lens 71 and a mask 73, and the lens 72 is disposed at a position where the distance f between the lens 72 and the fly eye lens 71 is equal to the distance between the lens 72 and the mask 73 (see Masato Shibuya, Makoto Uehara, "Illumination optical Device", Japanese Patent Publication No. Sho. 60-230629).
However, this method has the problems that the element length of the fly eye lens 71 becomes long, and efficiency is lowered since the sizes of illumination regions from the respective elements are different.
It is also proposed to realize similar effects by using a prism 75 as shown in FIG. 3 (see Japanese Patent Application No. Sho. 63-22131). However, in this method, the effect of coherence reduction is insufficient, and optical losses is high.
In principle, although it is also possible to obtain similar effects by using dispersion by refractive index, there has been a problem that in order to obtain a sufficient effect by a method of using normal dispersion by refractive index, an element becomes huge for coherence reduction.
Other than those, a number of methods of coherence control have been proposed. However, in a display, a microscope, or the like, any method has not been able to sufficiently reduce the speckle produced between an object to be illuminated and the naked eye. Further, in order to remove this speckle, it becomes necessary to carry out greater coherence control than with a projection exposure device such as for lithography.
That is, as shown in FIG. 4, an object 80 illuminated by illuminating light "all forms an image 83 on a screen 82 through a lens 81. Here, in the case where the illuminating light "a" is coherent light, the light receives random phase disturbance through the state of a rough surface of the object 80, the state of an optical surface of the lens 81, and the like, so that the image 83 on the screen 82 exhibits speckle effects.
Further, as schematically shown in FIG. 5, an image of an object formed on a screen through a lens and observed with an eye is equivalent to an image of an object 85 formed on a screen 87 through a lens 86 and formed on a retina 89 through an eyeball 88. That is, in this process, a random phase shift occurs on an optical path by disturbance of light at the screen 87 and the eyeball 88, and speckles are produced in this imaging process as well. Even if the speckles are not superimposed on the image on the screen 87, if there is spatial coherence on an image plane, secondary speckles are produced on the naked eye (retina 89).
A method such as a mirror oscillation or a rotation diffusion plate used in a projection exposure device on the basis of a lithography technique does not reduce the coherence, but merely moves speckles to average them. Thus, even if such a method is used, a remarkable effect on speckle produced on the naked eye is not obtained. In order to apply this method to a display or the like, there is only a method of oscillating a screen so that the positional relation between an object to be illuminated, such as a screen, and an eye is changed (see Eric G. Rawson, Antonio B. Nafarrate, Robert E. Nortone Joseph W. Goodman, "Speckle-free rear-projection screen using two close screens in slow relative motion", Journal of Optical Society of America, Vol. 66, No. 11, November 1976, pp 1290-1294). However, this is extremely inconvenient for practical use.
On the other hand, an optical fiber has been developed for mainly communication usage, and a glass material (glass fiber) containing quartz or the like as its main ingredient has been used for its constituent material. Besides, in order to avoid mode dispersion, the main purpose has been with the development of a single mode optical fiber.
In the-glass fiber, scattering increases in a visible short wavelength range, and its transmissivity is lowered. Thus, the application of an optical fiber to visible light has been restricted to a multi-mode optical fiber bundle (multi-mode fiber bundle) for illumination of a microscope, or the like, in which long transmission is not required. Especially in the case where the multi-mode optical fiber is used, the intensity distribution of emitted light becomes uniform, so that a complicated optical system such as a fly eye lens is not required, which is also a great merit.
In relation to this, recently, a plastic multi-mode optical fiber has been developed, and has attracted attention (see Takaaki Ishigure, Eisuke Nihei, and Yasuhiro Koike, "Graded-index polymer optical fiber for high-speed data communication", Applied Optics, Vol. 33, No.19, 1. July 1994, pp. 4261-4266).
The plastic fiber is inexpensive and lightweight as compared with a glass fiber, and has a feature that it has maximum transmission efficiency in a visible range. Further, multi-mode dispersion is also extremely large as compared with a normal glass fiber.
Besides, in recent years, a hollow waveguide for ultraviolet laser transmission has also been developed (see Preliminary collected papers for 58-th academic lecture at applied physics society, 3a-SR-18, Masaki Tsubokura, Yuichi Hashin, Uichi Kubo, "Improvement of a hollow waveguide for ultraviolet laser power transmission 11).
It has been known that the contrast of speckle is lowered by multi-mode dispersion in multi-mode optical fiber transmission, (see Masaaki Imai, "Fluctuation characteristic of an optical fiber and speckle", Optics, Vol. 8, No. 3, June 1979, p128-134).
That is, as shown in FIG. 6, in a multi-mode optical fiber 92 made of a core 93 and a clad 94, laser light 90 and laser light 91 having different modes have different transmission speeds. Thus, at the side of an emitting end 95 of the multi-mode optical fiber 92, light beams having different mode components from each other come to correspond to light beams incident at different times (t1, t2, and t3). Thus, if the extension due to this mode dispersion is longer than a coherence length, the coherence of the emitted light is reduced.
However, it is difficult to transmit laser light with sufficiently high intensity by only such a multi-mode optical fiber. Even in the case where such fibers are bundled, since laser light emitted from different optical fibers have coherence to each other, it is difficult to control the coherence. That is, to sufficiently reduce the speckle. Further, in order to practically apply this to illumination usage, it becomes necessary to use an optical fiber with high dispersion and high transmissivity in a visible range.
In view of such circumstances, in recent years, there has been proposed a method in which speckle is reduced by using a bundle of optical fibers (bundle fiber) with lengths different from each other by a length not less than a coherent length of an optical source (see D. Kohler, W. L. Seitz, T. R. Loree and S. D. Gardner, "Speckle reduction in pulsed-laser photographs", Optics Communications, 12, pp. 24-28, 1974, Benjamin Dingel and Satoshi Kawata, "Laser-diode microscope with fiber illumination", Optics Communications, 93, pp. 27-32, 1992, B. Dingel, S. Kawata, S. Minami, "Speckle reduction with virtual incoherent laser illumination using a modified fiber array", Optik, 3, 132-3136, 1993). Concerning this, the present applicant also proposed a technique (Japanese Patent Application No. Hei. 10-25646 filed Feb. 6, 1998).
However, including the case where speckle is reduced by using the bundle fiber, in general, in order to sufficiently reduce the contrast of speckle, it is necessary to sufficiently consider the number of divided light beams, the number of light sources, and the like.
For example, the degree at which speckle contrast is recognized by a human eye differs based on ambient brightness, color, differences among individuals, and the like. However, in general, a blur within 10 % on a screen is permissible, and if it is not larger than 5%, it cannot be recognized by a human eye. Although this is different between a moving picture and a still picture, or a monochrome image and a multicolor image, it is roughly a value of such a degree.